1. Field of the Invention
The present invention relates to sensors for reading magnetic flux transitions from magnetic media such as disks and tape. More particularly, the invention concerns a technique for resetting the magnetic orientation of one or more spin valves in a magnetoresistive read head.
2. Description of the Related Art
A magnetoresistive ("MR") sensor detects magnetic field signals by measuring changes in the resistance of an MR element, fabricated of a magnetic material. Resistance of the MR element changes as a function of the strength and direction of magnetic flux being sensed by the element. Conventional MR sensors operate on the basis of the anisotropic magnetoresistive ("AMR") effect, in which a component of the element's resistance varies as the square of the cosine of the angle between the magnetization in the element and the direction of sense or bias current flow through the element.
MR sensors are useful in magnetic recording systems where recorded data is read from a magnetic medium. In particular, the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of the magnetization of an MR head. This in turn causes a change in electrical resistance in the MR read head and a corresponding change in the sensed current or voltage.
A variety of magnetic multilayered structures demonstrate a significantly higher MR coefficient than an AMR sensor. This effect is known as the giant magnetoresistive ("GMR") effect. The essential features of these structures include at least two ferromagnetic metal layers separated by a nonferromagnetic metal layer. This GMR effect has been found in a variety of systems, such as iron-chromium (FeCr) and cobalt-copper (CoCu) multilayers exhibiting strong antiferromagnetic coupling of the ferromagnetic layers. The GMR effect is also found in essentially uncoupled layered structures in which the magnetization orientation in one of the two ferromagnetic layers is fixed or pinned. The physical origin is the same in all types of GMR structures: the application of an external magnetic field causes a variation in the relative orientation of the magnetizations of neighboring ferromagnetic layers. This in turn cases a change in the spin-dependent scattering of conduction electrons and thus the electrical resistance of the structure. The resistance of the structure thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes.
One specific application of GMR is the spin valve sensor. Spin valve sensors include a nonmagnetic conductive layer called a "spacer" layer, sandwiched between "pinned" and "free" ferromagnetic layers. The magnetization of the pinned layer is pinned 90.degree. to the quiescent magnetization of the free layer. Unlike the pinned layer, the free layer has a magnetic moment that freely responds to external magnetic fields, including those from a magnetic disk.
A spin valve sensor may be used to read data by directing a sense current through the free, spacer, and pinned layers of the sensor. The resistance of the spin valve sensor changes in proportion to rotation of the magnetic free layer (which moves freely) relative to the pinned layer (which is fixed in place). Such changes in resistance are detected and ultimately processed as playback signals.
In a typical spin valve MR sensor, the free and pinned layers have equal thicknesses, but the spacer layer is one half as thick as either of the free or pinned layers. An exemplary thickness of each of the free and pinned layers is 50 .ANG. and an exemplary thickness of the spacer layer is 25 .ANG..
As mentioned above, the magnetization of the pinned layer is pinned 90.degree. to the magnetization of the free layer. Pinning may be achieved by depositing the ferromagnetic layer to be pinned onto an antiferromagnetic layer to create an interfacial exchange coupling between the two layers. The antiferromagnetic layer may be constructed from a group of materials which include FeMn, NiMn, and NiO.
The spin structure of the antiferromagnetic layer can be aligned along a desired direction (in the plane of the layer) by heating beyond the "blocking" temperature of the antiferromagnetic layer and cooling in the presence of a magnetic field. The blocking temperature is the temperature at which the magnetic spins within a material lose their orientation. In other words, a material's blocking temperature is reached when exchange anisotropy vanishes because the local anisotropy of the antiferromagnetic layer, which decreases with temperature, has become too small to anchor the antiferromagnetic spins to the crystallographic lattice. The blocking temperatures of many antiferromagnetic materials ranges from about 160.degree. to 200.degree. C. Thus, when the blocking temperature of the antiferromagnetic material is exceeded, the spins of the antiferromagnetic layer lose their orientation causing the first ferromagnetic layer to no longer be pinned.
Unlike the pinned layer, the free layer has a magnetic moment that freely responds to external magnetic fields, including those from a magnetic disk. The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons are scattered by the interfaces of the spacer layer with the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal; when the magnetizations of the pinned and free layers are antiparallel, scattering is maximized. Due to changes in scattering, the resistance of the spin valve sensor changes in proportion to the cosine of the angle between the magnetizations of the pinned and free layers.
A number of U.S. patents disclose spin valve sensors. One patent, for example, shows a spin valve sensor in which at least one of the ferromagnetic layers is Co an alloy thereof, where the magnetizations of the two ferromagnetic layers are maintained substantially perpendicular to each other at zero externally applied magnetic field by exchange coupling of the pinned ferromagnetic layer to an antiferromagnetic layer. See, e.g., U.S. Pat. No. 5,159,513, assigned to International Business Machines Corp. Another patent discloses a basic spin valve sensor where the free layer is a continuous film having a central active region and end regions. The end regions of the free layer are exchange biased by exchange coupling to one type of antiferromagnetic material, and the pinned layer by exchange coupling to a different type of antiferromagnetic material. See, e.g., U.S. Pat. No. 5,206,590.
A read head employing a spin valve sensor, called a "spin valve read head", may be combined with an inductive write head to form a "combined" head. The combined head may have the structure of either a merged head, or a piggyback head. In a merged head a single layer serves as a shield for the read head and as a first pole piece for the write head. A piggyback head has a separate layer which serves as the first pole piece for the write head. In a magnetic disk drive an air bearing surface ("ABS") of a combined head is supported adjacent a rotating disk to write information on or read information from the disk. Information is written to the rotating disk by magnetic fields which fringe across a gap between the first and second pole pieces of the write head.
To read data, a sense current is directed through the free, spacer, and pinned layers of the sensor. The resistance of the spin valve sensor changes in proportion to relative rotation of the magnetic moments of the free and pinned layers. Such changes in resistance are detected and ultimately processed as playback signals.
Known spin valve sensors provide a number of benefits, most notably their significantly higher MR coefficient in comparison to AMR sensors. However, spin valves are sensitive to heating, which can disorient the magnetic spins in both antiferromagnetic and ferromagnetic films of the spin valve. This occurs whenever the heat source exceeds the blocking temperature of the antiferromagnetic films.
The chief sources of heat are electrostatic discharge and electrostatic overstress. Electrostatic discharge often ruins a sensor completely, whereas electrostatic overstress usually reduces the sensor's efficiency. These blocking temperatures can be reached by certain thermal effects during operation of the disk drive, such as an increase in the ambient temperature inside the drive, heating of the spin valve sensor due to the bias current, and rapid heating of the spin valve sensor due to the head carrier contacting asperities on the disk. In addition, magnetic disk drives are especially vulnerable to electrostatic discharge during the manufacturing process, such as during fabrication and assembly. If any of these thermal effects cause the spin valve sensor to exceed the antiferromagnet's blocking temperature, the magnetization of the pinned layer will no longer be pinned in the desired direction. This changes the spin valve sensor's response to an externally applied magnetic field, resulting in errors in data read from the disk.
A number of precautions are taken to avoid the dangers of heat-induced magnetic disorientation. For example, during the manufacturing process technicians can electrically ground themselves and their workpieces. Nonetheless, damage to spin valve sensors still occurs under some circumstances. Electric over stress can change pinned layer magnetization orientation. This is due to the fact that the heating by the current raises the temperature of the head near to the blocking temperature. Since the exchange field drops to near zero around the blocking temperature, antiferromagnetic layer spins around a neighboring ferromagnetic layer (pinned layer) magnetization will assume the direction of the field generated by the current. However, the field from the sense current is only of limited value (around only about 20 Oe). Therefore, if the pinned layer has coercivity in addition to the exchange field, and if the coercivity value is larger than the field from the current, then the field from the current will not be able to properly orient the pinned layer magnetization. Coercivity, in contrast to the exchange field, does not drop so strongly with temperature; as a result, coercivity can be fairly high, even at elevated temperatures. In addition, since the field from the current is non-uniform over the active area of the sensor it does not set the magnetization of the pinned layer over the entire pinned layer. As a result of these factors, electric overstress can severely diminish or disable the functionality of a spin value sensor.